After Sen. Joe Manchin’s bitter exchange with the Biden White House on Sunday, there’s increasing talk of Manchin leaving the Democratic Party. Mitch McConnell invites him to the GOP, but Axios reports that it’s more likely he’d become an independent but still caucus with the Democrats. How much difference would that make to the Senate? Would liberal Democrats still just be one or two votes short on party priorities like Build Back Better and voting rights?

Back in 2009 when Sen. Arlen Specter (R‑PA) switched parties, I did some research on the impact of modern party switching. There aren’t all that many examples, especially in the Senate.

The day after Republicans won control of the Senate in 1994, Sen. Richard Shelby of Alabama switched to the Republican Party. He had been a relatively conservative Democrat and had high‐​profile conflicts with President Bill Clinton, so the switch wasn’t a great surprise. But observers might be surprised to look back at what happened to Shelby’s voting record. According to the American Conservative Union, for the past eight years Shelby’s conservative voting percentage had ranged between 43 and 76. Even in 1994, as Shelby often found himself opposing the Clinton administration, the ACU gave him only a 55. But from 1995 to 2000, his ACU rating only once dipped below 90, and he scored a perfectly conservative 100 in 2000 and 2001 (even though Citizens Against Government Waste dubbed him the “King of Pork”). Meanwhile, the liberal Americans for Democratic Action had rated the Democratic Shelby 35 percent liberal in most years. As a Republican, however, ADA rarely found him more than 10 percent liberal. Shelby’s voting clearly changed when his party label did.

A few months after Shelby, Colorado Sen. Ben Nighthorse Campbell also switched from Democrat to Republican. He underwent a similar ideological migration. ACU rated him 12 and 25 in his first two years as a Democrat in the Senate, then 96 in the year of his switch. After that, his conservative score ranged from 72 to 96 until his retirement in 2005. His ADA score fell almost like stair steps — from 75 percent liberal in 1993 to an unusually low 30 the year he switched, then 45, 25, 25, 15, 5. According to Michael Barone, co‐​author of The Almanac of American Politics, Campbell switched his stands on partial‐​birth abortion, oil drilling in Alaska and assault weapons.

In 2001, Sen. Jim Jeffords of Vermont left the Republican Party and became an independent. Conservatives said he was already voting like a liberal Democrat. But that wasn’t quite right. Since he entered the Senate in 1989, his average ACU rating had been 27 — definitely the most liberal Republican, but not Ted Kennedy country. His ADA average was 58 — liberal for a Republican, but a long way from Vermont Democrat Pat Leahy. After the switch, Jeffords’ ACU rating plummeted: from 40 in 1999 and 36 in 2000 to 29 in the year of the switch to 6, 10, 4, 8 and 4 during the rest of his tenure. He and Leahy became twins.

And what about Specter, whose course I was trying predict in 2009? In 2008, as he prepared to face Pat Toomey in the Republican primary, Specter scored 45 on the ADA rating, lower than Susan Collins and Olympia Snowe of Maine. In 2010 Specter scored 90 on the ADA rating, the same as fellow Pennsylvanian Bob Casey, Rhode Island’s Jack Reed and Sheldon Whitehouse, and other liberal Democrats. His ACU ratings were even more revealing: between 40 and 43 in the three years before he switched, then 20 in 2009 and a perfect 0 in 2010. That was his last year in the Senate, as he lost the Democratic primary.

So it seems clear that party switchers do change their voting behavior. Why? 

First, the pressures from the party caucus obviously change. Surrounded by new colleagues, pressured to help the party deliver on its promises, the new guy finds himself going along with his new team. Of course, the real point here may be that the party‐​switcher has been liberated from the pressures in his former caucus that kept him from voting as he would have preferred.

Second, when an elected official switches parties in Washington, he also switches constituencies back home. Campbell’s political challenge as a Democrat was to persuade some 60 percent of the voters of Colorado, ranging from center to left, to support him. As a Republican, his goal became to get some 60 percent of Coloradans ranging from center to right to back him. It apparently worked: he won his 1998 reelection in a landslide. 

If Manchin becomes a Republican, and if he runs for reelection in 2024 at 77, he can assume he’ll lose many of the 30 percent of West Virginians who voted for Biden. But he would likely win by a larger margin than he did in 2018 as the last Democrat holding statewide office in the state.

How would that change his voting? Manchin is not all that conservative. His last three ACU ratings were 36, 32, and 26. His 2020 ADA rating was 75. That may reflect his real views as a lifelong Democrat from a Democratic family. But it may also be influenced by his being part of the Senate Democratic caucus, and thus likely to change if he switched parties.

But note: If Manchin declares himself an independent but remains a member of the Democratic caucus, then we may not see much change in his voting. After Sen. Joe Lieberman lost the Democratic primary in 2006 and ran as an independent, he stayed in the Democratic caucus, and his voting didn’t change much. It’s not clear what benefit Manchin would see in simply calling himself an independent. Becoming a Republican, though, would shake up the Senate. Biden and Sen. Chuck Schumer would no longer have even nominal control of the Senate, and Republicans would chair all the committees, presumably including Manchin at Energy and Natural Resources. One imagines that both parties are offering strong inducements to Manchin this week. Move the Pentagon to West Virginia, anyone?